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Pineal gland
The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles. The shape of the gland resembles a pine cone from which it derived its name. The pineal gland is located in the epithalamus, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of the thalamus join. The pineal gland is one of the neuroendocrine secretory circumventricular organs in which there does not exist the blood-brain barrier at the capillary level. Nearly all vertebrate species possess a pineal gland. The most important exception is a primitive vertebrate, the hagfish. Even in the hagfish, however, there may be a "pineal equivalent" structure in the dorsal diencephalon. The lancelet Branchiostoma lanceolatum, the nearest existing relative to vertebrates, also lacks a recognizable pineal gland. The lamprey (another primitive vertebrate), however, does possess one. A few more developed vertebrates have lost pineal glands over the course of their evolution. The results of various scientific research in evolutionary biology, comparative neuroanatomy, and neurophysiology have explained the evolutionary history of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor. In the epithalamus of some species of amphibians and reptiles, it is linked to a light-sensing organ, known as the parietal eye, which is also called the pineal eye or third eye. René Descartes believed the human pineal gland to be the "principal seat of the soul". Academic philosophy among his contemporaries considered the pineal gland as a neuroanatomical structure without special metaphysical qualities; science studied it as one endocrine gland among many. Etymology The word pineal, from Latin pinea (pinecone), was first used in the late 17th century to refer to the cone shape of the brain gland. Structure The pineal gland is a midline brain structure that is unpaired. It takes its name from its pinecone shape. The gland is reddish-gray and about the size of a grain of rice (5-8 mm) in humans. The pineal gland, also called the pineal body, is a part of the epithalamus, and lies between the laterally positioned thalamic bodies and behind the habenular commissure. It is located in the quadrigeminal cistern near to the corpora quadrigemina. It is also located behind the third ventricle and is bathed in cerebrospinal fluid supplied through a small pineal recess of the third ventricle which projects into the stalk of the gland. Blood supply Unlike most of the mammalian brain, the pineal gland is not isolated from the body by the blood-brain barrier system; it has profuse blood flow, second only to the kidney, supplied from the choroidal branches of the posterior cerebral artery. Nerve supply The pineal gland receives a sympathetic innervation from the superior cervical ganglion. A parasympathetic innervation from the pterygopalatine and otic ganglia is also present. Further, some nerve fibers penetrate into the pineal gland via the pineal stalk (central innervation). Also, neurons in the trigeminal ganglion innervate the gland with nerve fibers containing the neuropeptide PACAP. Microanatomy The pineal body consists in humans of a lobular parenchyma of pinealocytes surrounded by connective tissue spaces. The gland's surface is covered by a pial capsule. The pineal gland consists mainly of pinealocytes, but four other cell types have been identified. As it is quite cellular (in relation to the cortex and white matter), it may be mistaken for a neoplasm. Development The human pineal gland grows in size until about 1-2 years of age, remaining stable thereafter, although its weight increases gradually from puberty onwards. The abundant melatonin levels in children are believed to inhibit sexual development, and pineal tumors have been linked with precocious puberty. When puberty arrives, melatonin production is reduced. Symmetry In zebrafish, the pineal gland does not straddle the midline but shows a left-sided bias. In humans, functional cerebral dominance is accompanied by subtle anatomical asymmetry. Function Regulation of the pituitary gland Regulation of bone metabolism Clinical significance Calcification Tumors Other animals Society and culture History See also *Pineal gland cyst